1. Field of the Invention
The present invention relates to a transport container of a fuel assembly of a light water reactor such as a boiling water reactor (hereinafter, referred simply to as BWR), a pressurized water reactor (hereinafter, referred simply to as PWR) or the like, and to a method of transporting the fuel assembly thereof. In particular, the present invention relates to a fuel assembly transport container and a fuel assembly transport method, which can transport the fuel assembly itself or a fuel protective container housing the fuel assembly while fixedly supporting a motion of the fuel assembly or the fuel protective container.
2. Description of the Prior Art
A vibration generated in transporting a fuel assembly of a light water reactor such as BWR, PWR or other similar reactors, is a factor of causing wear in a metallic contact portion of the fuel assembly. In the fuel assembly, a spent fuel assembly has no problem as to somewhat of wear caused during transport because a waste disposal of the spent fuel assembly, reprocessing thereof and the like are carried out.
Therefore, there is no need of subjecting a transport container of the spent fuel assembly to specific vibration measures for preventing vibrations of the fuel assembly, and the spent fuel assembly may be transported in a state of being safely accommodated in the transport container. As a result, in order to house a plurality of spent fuel assembly, a transport container, which has a large capacity and is compact in its structure, has been used.
On the other hand, in the case of a transport container of a fuel assembly which is not used yet, since the fuel assembly is mounted to a reactor so that an operation of the reactor is carried out, it is very important that wear and damage should not be caused in a metallic contact portion or other similar portions of the fuel assembly which is not used yet by the vibration thereof during transporting the fuel assembly to the reactor, a store house or the like. So, when transporting the fuel assembly, a transport container of the fuel assembly is subjected to specific measures for preventing vibrations of the fuel assembly so that a reliability is maintained in the fuel assembly and a reactor using the fuel assembly.
For preventing a vibration of the fuel assembly, there is a need of housing the fuel assembly in a fuel protective container (also, called as an inner container of a fuel transport container) in a state that a motion of the fuel assembly is fixedly supported, and further, housing the fuel protective container housing the fuel assembly in a basket of the transport container while fixedly supporting a motion of the fuel protective container.
Here, FIGS. 25A and 25B show a conventional fuel protective container housing a fuel assembly in a state that the fuel assembly is fixedly supported.
A fuel assembly 101 is constructed in the following manner. Specifically, a plurality of fuel rods are tied up in a bundle with a metallic upper tie-plate 102 which has a relatively large-mass and is situated on an upper portion when the fuel assembly 101 is accommodated in a reactor, and with a metallic lower tie-plate 103 which has a relatively large-mass and is situated on a lower portion when the fuel assembly 101 is accommodated in a reactor. The lower tie-plate 103 has step portions tapered toward the crosswise inner peripheral side surfaces 106c, 106c which are opposite each other, described hereinafter.
This fuel assembly 101 has a square pillar shape having a square shape in its lateral cross section, and has a length of one side of the square cross section is substantially 4 m in a longitudinal direction of the fuel assembly 101. Further, bundled fuel rods (fuel rod group) constituting the fuel assembly 101 are supported by means of a fuel spacer 104 with a predetermined interval.
A fuel protective container 105 comprises a container main body 106 having a substantially U shape in its lateral cross section, a cap member 107 which is detachably mounted on an upper portion (opening portion) of the container main body 106 which is transporting so as to cover the opening thereof, and protective members 108a.about.108d. The protective members 108a, 108b, 108c and 108d are formed along a bottom surface 106a of the container main body 106 along the horizontal direction when the container main body 106 is transported, longitudinally inner peripheral side surfaces 106b; 106b facing each other, crosswise inner peripheral side surfaces 106c; 106c which are opposite each other, and a lower surface 107a on the container main body side of the cap member 107, respectively. The fuel assembly 101 is accommodated in a fuel assembly housing space defined by the container main body 106 of the fuel protective container 105 and the cap member 107 so that the longitudinal direction of the fuel assembly 101 is parallel to the aforesaid horizontal direction during the transport of the container main body 106.
In order to prevent a vibration when transporting the fuel protective container 105 in which the fuel assembly 101 is housed, several sets of transport (fastening) separators 110 are interposed between the fuel spacers 104, between the fuel spacer 104 and the upper tie-plate 102, and between the fuel spacer 104 and the lower tie-plate 103. These separators are arranged so that gaps between the separators and the protective members 108b mounted on the longitudinal inner peripheral side surfaces 106b; 106b are formed.
That is, after the fuel assembly 101 is housed in the container main body 106, when the opening side upper portion of the container main body 106 is covered by the cap member 107 so as to be closed, the fuel assembly 101 is pressed down along a up and down direction (vertical direction) during the transport of the fuel assembly 101 by a fastening force of the cap 107 to the bottom surface 106a of the container main body 106 via the transport separator 110, and thus, is fixedly restricted therein. The fuel assembly 101 housed integrally with the fuel protective container 105 is transported while being fixedly supported by the fastening force via the transport separators arranged between the bottom surface 106a of the container main body 106 and the cap 107.
However, in the aforesaid conventional fuel protective container 105 in which the fuel assembly 101 is fixedly supported, the fuel assembly 101 is merely fixedly supported by fastening the fuel assembly 101 from the vertical direction. As shown in FIGS. 25A and 25B, the fuel assembly 101 is not clamped in the horizontal direction along the crosswise direction. For this reason, the gap still exists between both sides of the fuel assembly 101 and the protective member 108b formed on the longitudinal inner peripheral side surfaces 106b, 106b of the container main body 106.
As a result, there is the possibility that the fuel assembly 101 slides and moves on the protective member 108a formed on the bottom surface 106a of the container main body 106 along the aforesaid crosswise (lateral) direction.
In this case, as a power of resistance to a relatively sliding motion between the fuel assembly 101 and the protective member 108a formed thereon, there are recited the own weight of the fuel assembly 101 and a frictional force between the fuel assembly 101 and the protective member 108a based on a fastening force by the cap 107.
However, in the above power of resistance, concerning the fastening force by the cap member 107 recited as the frictional force, since the fastening portion is the transport separator 110 inserted into the fuel assembly 101, a compressive rigidity is small. When a great fastening force is applied on the transport separator 110, there is the possibility that the fuel assembly 101 is deformed; for this reason, a satisfied fastening force has not been provided by the cap member 107 on the transport separator 110. Therefore, concerning the frictional force resulting from the fastening force, a satisfied frictional force capable of preventing the sliding motion of the fuel assembly 101 has not been provided.
Consequently, because a tightly restricting force of the fuel assembly 101 is short in the horizontal direction along the longitudinal direction with respect to the fuel protective container 105, there has arisen a problem that the fuel assembly 101 moves (vibrates) while sliding in the fuel protective container 105 according to a vibration of the horizontal direction of a relatively high acceleration during the transport of fuel protective container 105.
In addition, a fastening force to the fuel assembly 101 is short in the horizontal direction along the longitudinal direction (axial direction) of the fuel assembly 101. Therefore, for example, in the case where a mixed-oxide fuel (MOX) assembly mixing a plutonium oxide (PuO.sub.2) and an uranium oxide (UO.sub.2) is used as the fuel assembly, during transport of the MOX fuel assembly, the MOX fuel assembly 101 is exothermic, and then, an elongation difference is caused due to a difference in thermal expansion between the MOX fuel assembly 101 and the fuel protective container 105. For this reason, a relatively positional shift is generated between the MOX fuel assembly 101 and the fuel protective container 105. In addition, a gap is defined between both end portions along the longitudinal direction (axial direction) of the MOX fuel assembly 101 and both side surfaces 106c of the fuel protective container 105 and between the MOX fuel assembly 101 and the bottom surface 106a of the fuel protective container 105.
As a result, similar to the aforesaid case of the horizontal direction along the crosswise direction, there is the possibility that the fuel assembly 101 slides and moves (vibrates) on the protective member 108a formed on the bottom surface 106a of the container main body 106 along the longitudinal direction according to a vibration of the horizontal direction of relatively high acceleration which arises from transporting the fuel protective container 105.
Moreover, in the conventional fuel protective container 105 in which the fuel assembly 101 is fixedly supported, since the fuel assembly 101 is fixedly supported by means of the transport separators 110 located between the fuel spacers 104, between the fuel spacer 104 and the upper tie-plate 102, and between the fuel spacer 104 and the lower tie-plate 103, a tightly fixing force is short in the attachment portions of the upper tie-plate 102 and the lower tie-plate 103 on both ends of the fuel assembly 101 in the longitudinal direction thereof. Therefore, resulting from mass of the upper tie-plate 102 and the lower tie-plate 103, there is the possibility that a remarkably different vibration is generated between the upper tie-plate 102 and the protective barrier 106 and between the lower tie-plate 103 and the same as compared with a vibration in the central portion of the fuel assembly 101 according to the aforesaid vibration of the horizontal direction during transport of the fuel protective container 105.
As described above, because the tightly fixing force in the horizontal direction is short or the tightly fixing force on portions locating the upper and lower tie-plates 102 and 103 is short, the fuel assembly 101 has slid and vibrated in the fuel protective container 105 housing the fuel assembly 101. This sliding vibration of the fuel assembly 101 causes a problem of accelerating a wear of the metallic contact portion of the bundled fuel rods group.
Furthermore, in the conventional fuel protective container 105 in which the fuel assembly 101 is fixedly supported, the fuel assembly 101, that is, the own weight of fuel rods group is supported by the transport separators 110. As a result, most of the own weight of fuel rods group are supported by a row of the fuel rods (the lowest row) which is closest to the bottom surface 106a of the fuel protective container 105 in the fuel rod groups.
For this reason, in a transport process of the fuel assembly 101, when a transport container housing the fuel assembly 101 is loaded and unloaded with the use of a crane (hoist) or other similar machines, in the case where an instantaneous force having a relatively high acceleration is applied to the fuel assembly 101, there is the possibility that the fuel rods situated on the lowest row are plastically deformed. This causes a problem that a loading and unloading condition during transport of the fuel assembly 101 must be strictly limited.
In particular, in a future fuel assembly, there is a tendency for a diameter of a fuel rod to be shortened. For this reason, there is the possibility that the loading and unloading restraint condition during the fuel assembly transporting process becomes further strict in future. Thus, it has been desired to present a proposal to immediately solve the above problem according to the deformation of the fuel assembly.
On the other hand, the fuel assembly has long one side whose length is substantially 4 m in the longitudinal direction thereof; for this reason, vibration is not sufficiently prevented only by fixedly supporting both side portions of the fuel protective container in the longitudinal direction thereof. Therefore, in order to fixedly support the fuel protective container housing the fuel assembly in a basket of a transport container, there is a need of fixedly supporting an intermediate portion of the fuel protective container in the longitudinal direction thereof. However, specific fixedly supporting means for protecting the aforesaid fuel protective container has not been conventionally developed.
Especially, the case of transporting the transport container which houses a plurality of fuel protective containers in the basket of the transport container, the fixedly supporting means basically needs to be provided for each fuel protective containers. However, conventionally, there is no existence of a small-size fixedly supporting means having a small spatial occupancy, and a spatial ratio occupied by the fixedly supporting means is large. This is the greatest factor of obstructing a development of a compact and large-capacity fuel transport container.
Further, in the case where the MOX fuel assembly is used as the fuel assembly, since the MOX fuel assembly is exothermic during the transport of the MOX fuel assembly so that a temperature of the fuel protective container 105 becomes high, fixedly supporting means needs to be provided in order to maintain a high reliability under such a high temperature condition. However, there is a problem that fixedly supporting means having a high reliability under the high temperature condition has not been developed conventionally.
Furthermore, according to the prior art, a plurality of fuel protective containers are fixedly supported in the basket of the transport container for each fuel protective container. For this reason, when the plurality of fuel protective containers are fixedly supported, manpower and time is much spent in accordance with the number of the fuel protective containers. Therefore, there has been strongly desired a development of a transport container having fixedly supporting means which is capable of reducing manpower and easily and fixedly supporting a plurality of fuel protective containers in a basket of the transport container in a short time.